Question

try the formation of image by diff in different situations using the concave lens

Answer 1

When an object is placed at infinity, the two rays AO and BD running parallel to the principal axis get refracted at point O and D respectively and get diverged along the directions OX and DY respectively. These diverged rays i.e. OX and DY appears to intersect each other at the principal focus of the concave lens after extending back by dotted lines. Therefore, in case of concave lens when the object is placed at infinity the image is formed at the principal focus, highly diminished (point sized), virtual and erect.